Unable to play mp4 videos. I used Real player downloader to download videos from youtube. It downloaded it in .mp4 format. But I cudn't play this in Real player, Quick time and Elmedia player.. In short.. how can I download videos from you tube and play?

Since you're already downloading successfully, the problem is playing the downloaded file. Go here http://www.videolan.org/vlc/download-macosx.html for VLC, which will play nearly any video file type.

While I don't do video editing, I do use two downloading extensions in Firefox and both offer multiple video formats to chose from when downloading. One is "DownloadHelper" and the other is "Flash Video Downloader." While each has its own website, you should download them from the Firefox extension website since what's presented there has been vetted by Mozilla. The links are http://www.flashvideodownloader.org/ and https://addons.mozilla.org/en-US/firefox/addon/downthemall/ so you can see if they'll offer what you need.